GIS Definitions: Q-S
Definitions of some commonly used GIS (Geographic Information System) terms.
Q R S
Full glossary: A-D
- quadrangle (quad)
- See topographic
- A spatial index which
recursively decomposes a data set (e.g., image) into square cells
of different sizes until each cell has a homogeneous value. Quadtrees
are often used for storing raster data. See also spatial
- See map query.
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- A cellular data structure
composed of rows and columns for storing images. Groups of cells with
the same value represent features. See also grid.
- See route attribute table.
- Relational database management
system. A database management system with the ability to access data
organized in tabular files that can be related to each other by a
common field (item). An RDBMS has the capability to recombine the
data items from different files, providing powerful tools for data
usage. See also relate.
- real numbers
- Decimal numbers (e.g.,
3.1417, 0.25, 1.8992, 6.0).
- 1. In an attribute table,
a single 'row' of thematic descriptors. In SQL terms, a record is
analogous to a tuple.
2. A logical unit of data in a file. For example, there is one record
in the ARC file for each arc in a coverage.
- The process by which an
image or grid is converted from image coordinates to real-world coordinates.
Rectification typically involves rotation and scaling of grid cells,
and thus requires resampling of values.
- The capability to ensure
that changes to one table that affect other tables are transmitted
to those other tables. For example, a table will not be given a foreign
key value that does not exist as a primary
key in another table.
- A coverage feature class
used to represent a spatial feature as one or more polygons. Many
regions can be defined in a single coverage. Regions have attributes
that describe the geographic feature they represent.
- registered table
- A DBMS table which is part of an ArcStorm database is said to be 'registered' with the database.
- An operation that establishes
a temporary connection between corresponding records in two tables
using an item common to both (i.e., relate key). Each record in one
table is connected to those records in the other table that share
the same value for the common item. Compare with relational join.
- The common set of columns
used to relate two attribute tables. See also relate,
key and foreign
- See table.
- relational database
- A method of structuring
data as collections of tables that are logically associated to each
other by shared attributes. Any data element can be found in a relation
by knowing the name of the table, the attribute (column) name, and
the value of the primary key. See also relate,
relate key, and relational
- The operation of relating
and physically merging two attribute tables using their common item.
- remote sensing
- Acquiring information
about an object without contacting it physically. Methods include
aerial photography, radar, and satellite imaging.
- The process of reducing
image data set size by representing a group of pixels with a single
pixel. Thus, pixel count is lowered, individual pixel size is increased,
and overall image geographic extent is retained. Resampled images
are "coarse" and have less information than the images from which
they are taken. Conversely, this process can also be executed in the
reverse. In ArcInfo, the GRID function RESAMPLE supports resampling
of raster data using Cubic Convolution, Bilinear Interpolation, Nearest
Neighbor Assignment, and custom "Nearest Data" assignment methods.
- 1. Resolution is the accuracy
at which a given map
scale can depict the location and shape of geographic features.
The larger the map scale, the higher the possible resolution. As map
scale decreases, resolution diminishes and feature boundaries must
be smoothed, simplified, or not shown at all. For example, small areas
may have to be represented as points.
2. Distance between sample points in a lattice.
3. Size of the smallest feature that can be represented in a surface.
4. The number of points in x and y in a grid or lattice (e.g., the
resolution of a U.S. Geological Survey one-degree DEM is 1201 x 1201
- To return a database to
a previous state by undoing all changes made since the given time.
ArcStorm provides a restore mechanism.
- RMS error
- Root mean square error.
A measure calculated when registering a map to a digitizer, indicating
the discrepancy between known point locations and their digitized
locations. The lower the RMS error, the more accurate the digitizing
or transformation. See also tic.
- To cancel any changes
to a database made during the current transaction. Compare with commit.
- A feature class in ArcInfo
that is part of the route-system data model used to represent linear
features. Routes are based on an arc coverage and are defined as an
ordered set of sections. Because sections represent the portion of
an arc used in a route, routes do not have to begin or end at nodes.
The route attribute table (RAT)
stores route attributes. See also route-system
and route measure.
- route attribute table (RAT)
- Route attribute table.
An RAT stores route attributes. There is one RAT for each route-system
in a coverage. In addition to user-defined attributes, an RAT contains
a sequence number and feature identifier for each route. See also
SEC, and feature
- A location along a route,
defined as a measure from a start point, where measures include distance,
time, milepost, address range. Measures are useful for locating events
along a route. There may be more than one start or end within a route,
in which case, the measures may not be unique within a route-system.
For example, there are many places that are within a one-minute response
time from a fire station.
- A collection of routes
representing separate instances of a common linear entity, for example,
all school bus routes in a city. A single line coverage can contain
many route-systems, differentiated by name. For example, a road coverage
can contain a bus route-system, a highway route system and a pizza
delivery route-system. Both an RAT
and an SEC exist for each route
system. See also event.
- 1. A record in an attribute
table. The horizontal dimension of a table composed of a set of columns
containing one data item each.
2. A horizontal group of cells in a grid, or pixels in an image.
- A Remote Procedure Call
(RPC) is a communication mechanism which allows one UNIX process to
communicate with another UNIX process. These communicating processes
can be on different computers over a network. ArcStorm servers and
clients use RPCs to communicate with each other.
- rubber sheeting
- A procedure to adjust
coverage features in a nonuniform manner. Links
representing from- and to-locations are used to define the adjustment.
- run-length encoding
- A data compression technique
for storing raster or gridded data. Run-length encoding stores data
by row. If two or more adjacent cells in a row have the same value,
the 'run' is recorded, as opposed to recording an individual value
for each cell. The more adjacent columns having the same value, the
greater the compression.
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- satellite image
- A picture of the earth
taken from an earth-orbital satellite. Satellite images may be produced
photographically or by on-board scanners (e.g., MSS).
- See map
- scale bar
- A map element that shows
the map scale graphically.
- The process of capturing
data in raster format with a
device called a scanner. Some scanners also use software to convert
raster data to vector data.
- scratch file
- A temporary file holding
intermediate data during an operation, such as when calculating arc
intersections, or building feature topology.
- Spatial Data Transfer
Standard/Topological Vector Profile. A United States Federal standard
designed to support the transfer of different types of geographic
and cartographic spatial data. This standard specifies a structure
and content for spatially referenced data in order to facilitate data
transfer between dissimilar spatial database systems. TVP addresses
a wide variety of vector data types, models, and structures, as well
as associated attribute data. Also known as Federal Information Processing
Standard (FIPS) 173.
- Section table for the
section feature class in a coverage. The SEC holds attributes about
sections. In addition to user-defined attributes, the SEC contains
information on both the route number and arc number to which the section
belongs, the starting and ending positions expressed as percentages
of the arc length, starting and ending positions expressed as measures
along the route, an internal sequence number and a section feature
identifier. See also feature
- A feature class in ArcInfo
that is a component of the route
system data model used to implement routes.
They form the infrastructure of route-systems much like arcs form
the infrastructure of polygons. Sections are the arcs, or portions
of arcs, used to define each route. See also SEC.
- A line on a surface defining
the position of a profile.
- selection coverage
- A coverage whose area
overlaps that of a map library. It identifies the area to extract
data from, or insert data into, a map library.
- A process which can set
its effective user to super-user (root). That is, although any user
can run the process, the process can then execute operations which
require root privileges. One example of a setuid process is the ArcStorm
- A pattern used to shade
polygons in ArcInfo. Shade symbol patterns include crosshatch, repeating,
and solid fill.
- Standard interchange format,
a spatial data exchange format. A standard or neutral format used
to move graphics files between computer systems.
- Refers to a level of coordinate
accuracy based on the number of significant digits that can be stored
for each coordinate. Single-precision numbers store up to 7 significant
digits for each coordinate, retaining a precision of 5 meters in an
extent of 1,000,000 meters. ArcInfo data sets can be stored as either
single- or double-precision coordinates. See also double
- sliver polygon
- A small areal feature
commonly occurring along the borders of polygons following the topological
overlay of two or more coverages.
- A measure of change in
surface value over distance, expressed in degrees or as a percentage.
For example, a rise of 2 meters over a distance of 100 meters describes
a 2% slope with an angle of 1.15. Mathematically, slope is referred
to as the first derivative of the surface.
- Systems network architecture.
Networking protocol popular in IBM environments.
- The process of moving
a feature to coincide exactly with coordinates of another feature
within a specified snapping distance, or tolerance.
- A phonetic spelling (up
to six characters) of a street name, used for address matching. Each
of the 26 letters in the English alphabet are replaced with a letter
in the soundex equivalent:
English: A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z
Soundex: A B C D A B C H A C C L M M A B C
R C D A B W C A C
Where possible, geocoding uses a soundex equivalent of street names
for faster processing. During geocoding, initial candidate street
names are found using soundex, then real names are compared and verified.
See also geocode.
- The process of modeling,
examining, and interpreting model
results. Spatial analysis is useful for evaluating suitability and
capability, for estimating and predicting, and for interpreting and
understanding. There are four traditional types of spatial analysis:
overlay and contiguity analysis, surface analysis, linear analysis,
and raster analysis.
- spatial data
- Information about the
location and shape of, and relationships among, geographic features,
usually stored as coordinates and topology.
- spatial feature
- See geographic
- A means of accelerating
coverage drawing, spatial selection, and feature identification by
generating feature-based indexes for one or more feature classes of
- spatial interaction
- An analytical technique
that estimates the number of interactions
occurring between an origin and destination locations. The number
of interactions is based on the properties of the origin to produce
a trip (production),
the destination's attractiveness
and the impedance
of the link between the two locations. The goal of spatial interaction
modeling is to be able to model and predict the number of interactions
occurring between populations for a particular type of activity such
- Analytical procedures
applied with a GIS. There are three categories of spatial modeling
functions that can be applied to geographic features within a GIS:
(1) geometric models, such as calculating the Euclidean
distance between features, generating buffers, calculating areas and
perimeters, and so on;
(2) coincidence models, such as topological
(3) adjacency models (pathfinding, redistricting, and
All three model categories support operations on spatial data such
as points, lines, polygons, tins, and grids. Functions are organized
in a sequence of steps to derive the desired information for analysis.
See also model
- spatial order
- An index assigned to features
based on their relative closeness in two-dimensional space.
- spatial query
- See map
- 1. An overshoot line created
erroneously by a scanner and its rasterizing software.
2. An anomalous data point that protrudes above or below an interpolated
surface representing the distribution of the value of an attribute
over an area.
- A mathematical curve used
to smoothly represent spatial variation. A spline operation inserts
vertices to create a curve in an arc. See also grain
tolerance and densify.
- Structured Query Language.
A syntax for defining and manipulating data from a relational database.
Developed by IBM in the 1970s, it has become an industry standard
for query languages in most relational database management systems.
- An international standards
effort, sponsored by ISO to extend SQL to support multimedia applications,
including access and manipulation of geographic data.
- Standardized raster graphic,
a digital representation of a map or chart, which is captured by automatic
digitization (scanning), stored on a digital storage media, and displayed
on a raster screen or raster plotter; obtained by a regular scan of
a paper map or chart or repromat. It consists of a raster data set
of RGB intensities or colour (sic) codes.
- station file
- An AML file containing
commands needed to establish the environment for graphic display and
graphic input. Typically, station files contain commands that define
the DISPLAY device, the AML &TERMINAL device, the type of DIGITIZER,
if any, and the method to be used for COORDINATE input.
- Stops are locations visited
in a path
they may represent customers on a delivery route or cities in a highway
system. Stops and stop attributes are maintained in INFO files referred
to as stops files.
- stop impedance
- The time it takes for
a stop transfer to occur.
This is used to compute the impedance
of a path
- The number of things or
packages transferred at a stop. This is used to find the total number
of transfers on a path or tour.
- A series of alphanumeric
characters of any length enclosed by quotes.
- A special feature
class in a coverage which allows many features of the same class
to be defined. Annotation, region, route-system, and section are types
of subclasses. For example, a road coverage may have three route-systems
stored as subclasses for mail delivery, street cleaning, and garbage
- The availability of services
or goods at centers.
An example of a type of supply is the availability of milk at grocery
stores, cars at a car dealer, or the number of movie screens at a
- A geographic phenomenon
represented as a set of continuous data, such as elevation or air
temperature over an area. A clear or sharp break in values of the
phenomenon (breaklines) indicates a significant change in the structure
of the phenomenon (e.g., a cliff), not a change in geographic feature.
Surfaces can be represented by models built from regularly or irregularly
spaced sample points on the surface. See also surface
- Digital abstraction or
approximation of a surface. Because a surface contains an infinite
number of points, some subset of points must be used to represent
the surface. Each model contains a formalized data structure, rules,
and x,y,z point measurements that can be used to represent a surface.
The TIN software package supports two data models for representing
- A relational database
management system to which ArcInfo has access through the DATABASE
- A graphic pattern used
to represent a feature. For example, line symbols represent arc features;
marker symbols, points; shades symbols, polygons; and text symbols,
annotation. Many characteristics define symbols, including color,
size, angle, and pattern. See also text
symbol, shade symbol,
- symbol environment
- Defines the types of map
symbols and their characteristics during a graphic display session
in ArcInfo. There are four types of active map symbols: line, marker,
shade, and text.
- Tables which contain information
about a database, such as the data dictionary and database transactions.
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Glossary pages: A-D